Re: [PATCH v9 16/41] KVM: guest_memfd: Zero page while getting pfn

On 7/29/2026 8:35 AM, Ackerley Tng via B4 Relay wrote:
> From: Ackerley Tng <[email protected]>
> 
> Move the folio initialization logic from kvm_gmem_get_pfn() into
> __kvm_gmem_get_pfn() to also zero pages if the page is to be used in
> kvm_gmem_populate().
> 
> With in-place conversion, the existing data in a guest_memfd page can be
> populated into guest memory through platform-specific ioctls.
> 
> Without first zeroing the page obtained using __kvm_gmem_get_pfn(), it
> might contain uninitialized host memory, which would leak to the guest if
> the populate completes.
> 
> guest_memfd pages are zeroed at most once in the page's entire lifetime
> with guest_memfd, and that is tracked using the uptodate flag.
> 
> Zeroing the page in __kvm_gmem_get_pfn() is chosen over zeroing in
> kvm_gmem_get_folio() since other flows, such as a future write() syscall,
> can get a page, write to the page and then set page uptodate without
> zeroing.
> 
> This aligns with the concept of zeroing before first use - the other place
> where zeroing happens is in kvm_gmem_fault_user_mapping().
> 
> Don't mark the page uptodate again after populating, since the page would
> already be marked uptodate before the post_populate() call.
> 
> Reviewed-by: Fuad Tabba <[email protected]>
> Tested-by: Shivank Garg <[email protected]>
> Signed-off-by: Ackerley Tng <[email protected]>

Reviewed-by: Xiaoyao Li <[email protected]>

> ---
>   virt/kvm/guest_memfd.c | 12 +++++-------
>   1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 505bb6747620b..ea2752989f8bd 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -1078,6 +1078,11 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
>   		return ERR_PTR(-EHWPOISON);
>   	}
>   
> +	if (!folio_test_uptodate(folio)) {
> +		clear_highpage(folio_page(folio, 0));
> +		folio_mark_uptodate(folio);
> +	}
> +
>   	*pfn = folio_file_pfn(folio, index);
>   	if (max_order)
>   		*max_order = 0;
> @@ -1107,11 +1112,6 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>   		goto out;
>   	}
>   
> -	if (!folio_test_uptodate(folio)) {
> -		clear_highpage(folio_page(folio, 0));
> -		folio_mark_uptodate(folio);
> -	}
> -
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
>   	if (kvm_gmem_is_private_mem(file_inode(file), index))
>   		r = kvm_arch_gmem_make_private(kvm, gfn, *pfn,
> @@ -1171,8 +1171,6 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
>   	}
>   
>   	ret = post_populate(kvm, gfn, pfn, src_page, opaque);
> -	if (!ret)
> -		folio_mark_uptodate(folio);
>   
>   out_put_folio:
>   	folio_put(folio);
>